In LTE systems, a scheduler resides in the base station and is responsible for dynamically assigning transmission resources to mobile terminals. Generally, the base station makes scheduling decisions for each one-millisecond transmission time interval, and sends scheduling information to the scheduled mobile terminals for each interval. To reduce the signaling overhead involved with the transmission of this scheduling information, semi-persistent scheduling is used in some circumstances, particularly when the traffic associated with a particular service (e.g., voice-over-IP, or VOIP) is relatively predictable and characterized by regular transmissions of small payloads. Thus, a semi-persistent scheduling grant indicates to a mobile terminal that it has been allocated a particular quantity and set of uplink resources, and that this allocation applies to every n-th subframe until the terminal is notified otherwise.
To perform its scheduling tasks efficiently, the scheduler needs information about each mobile station's current buffer status, i.e., whether each mobile station has data queued for transmission, and, if so, how much. In LTE, buffer status reports are provided to the base station by scheduled mobile stations as part of the control element headers associated with an uplink transport block transmission. Buffer status reports can be triggered for various reasons, including: a change in buffered data priority in the mobile station (e.g., data having a higher priority than any currently buffered data “arrives” at the transmission buffer); a change in serving cell; and/or expiration of a periodic timer. In addition, when the amount of padding that would be required to make an uplink transport block fit the scheduled transport block size exceeds the size of a buffer status report, a current buffer status report is sent to use the resources that would otherwise be wasted.
The more precise and up-to-date the buffer status information is, the more accurate the scheduling decisions can be. Of course, the buffer status reports provided to the base station by the mobile stations consume system resources as well. Thus, the various buffer status reporting schemes standardized for LTE and other wireless networks reflect a balance between the overhead introduced by the reporting itself and the efficiency gains achieved with the more precise and timely scheduling of resources that is facilitated by the reports.
Ideally, it would be useful to have buffer information for each logical channel applicable to a given mobile station. Of course, this would require more overhead. Accordingly, logical channels are grouped into logical-channel groups (LCGs) in LTE systems, and buffer status reporting is performed per group. Thus, the buffer-size field in a buffer-status report indicates the total amount of data buffered for transmission for all logical channels in a given LCG (or for four LCGs, in the case of a “long” buffer status report).
LTE base stations are also responsible for ensuring that the necessary quality-of-service (QoS) is met for each radio bearer that it handles. A radio bearer corresponds to an end-to-end IP packet flow between a mobile station and a gateway in the public data network, or PDN. (In implementations of the LTE protocol, the radio link control layer, or RLC, provides services to upper layers of the protocol stack at the radio bearer level. The RLC receives services from the medium access control layer, or MAC, at the level of logical channels. Thus, each logical channel corresponds to a particular radio bearer, which in turn has a one-to-one correspondence to an end-to-end IP flow.) Each bearer has a defined QoS, which is identified by a QoS class identifier, or QCI. The QCI label for a given bearer determinates how it is handled by the base station with regards to priority, packet delay budget, acceptable packet loss rate, and the like. LTE specifies ten different QCIs, each of which is suitable for one of a wide variety of types of services, ranging from conversational voice and conversational video to real-time gaming and Internet browsing.
In order to assess the real-time performance of the network, for support of load balancing and other “tuning” of an LTE network, 3GPP has specified a number of performance measurements that can be made from time to time. All LTE base stations must be capable of making these measurements, which include, for example, measurements of physical resource block (PRB) usage, packet delay, and packet loss. With these measurements, network operators can get a detailed picture of the throughput and performance of the system at any given time.
One of the required measurements specified by 3GPP is an estimate of the “Number of Active UEs per QCI.” “UE,” or “user equipment,” is 3GPP terminology for the mobile terminal equipment. In the present application, “UE,” “mobile terminal,” and “mobile station” are used interchangeably unless the context clearly indicates otherwise.) The “Number of Active UEs per QCI” measurement provides an estimate of the number of users sharing the same resources in the system; this estimate may be used provide a detailed view of the throughput and performance of the system at any given time. In early versions of the standards for these measurements, it is explained that the Number of Active UEs per QCI refers to the number of mobile stations for which there is currently buffered data for uplink transmission. This quantity is measured separately for each quality-of-service (QoS) classification for radio traffic. More particularly, the Number of Active UEs per QCI is defined as:
                                          M            ⁡                          (                              T                ,                qci                ,                p                            )                                ⁢                      ⌊                                                            ∑                                      ∀                                                                                  ⁢                    i                                                                                                          ⁢                                  N                  ⁡                                      (                                          i                      ,                      qci                                        )                                                                              I                ⁡                                  (                                      T                    ,                    p                                    )                                                      ⌋                          ,                            (        1        )            where M(T, qci, p) is the average number of active UEs in the uplink per QoS Class Identifier, N(i, qci) is the number of UEs for which there is buffered data for a data radio bearer (DRB) having a traffic class of QCI=qci at sampling instance i, p is a sampling period length, I(T, p) is the total number of sampling occasions during time period T.
The 3GPP specifications acknowledge that this quantity is only an estimation, stating that the number of UEs for which there is buffered data is expected to be based on Buffer Status Reporting, analysis of received data and progress of ongoing HARQ transmissions. Finally, the specifications note that when CQI cannot be determined at the time of the sampling occasion, it is expected that CQI is determined after successful reception of data. Thus, the current definitions leave details of a practical implementation unspecified, as the exact estimation process is not defined. In addition, the need in some cases to determine the QCI only after the successful reception of data will generate complex interaction between future data reception and a current sampling instance—in practice, data not yet received may affect a current estimate of the number of active UEs.